1. Technical Field
This invention relates to a side-band generator, and more particularly to a side-band generator which may be used in tissue characterization applications and which generates upper and lower side-band signals.
2. Discussion
Side-band generators are generally used in a plurality of electrical communication applications. One use of these side-band generators is associated with the detection of acoustical signals generated in a non-linear media, such as tissues. In this application, a high frequency sinusoidal ultrasound wave (i.e., a carrier) is typically transmitted, along with a low frequency sinusoidal wave (i.e., a modulator), into a sample of tissue which is desired to be studied relative to the health associated therewith. The high frequency and low frequency signals then acoustically interact within the tissue and the resultant mixing of the signals, when detected by an ultrasonic transducer, produces a characteristic electrical signature signal which is then used to properly characterize the tissue sample.
This electrical signature signal contains both an upper side-band component (having a frequency associated therewith being equal to the sum of the frequencies of the carrier and modulator signals) and a lower side-band component (having a frequency associated therewith, being equal to the difference of the frequencies of the carrier and modulator signals). In order to properly generate this signature, the upper or lower side-band signals must first be separated from the resultant electrical signal. A current methodology associated with this electrical signal characterization incorporates the use of a lock-in amplifier. While this lock-in amplifier is useful in the characterization of the electrical signature signal, it requiers the generation of the side-band frequency of interest (upper or lower) as a reference input thereto. It is useful that this side-band be generated from the auxiliary ("sync") output apertures of the carrier and modulator generators. The waveforms from these "sync" outputs are typically square waves of fixed amplitude.
Techniques which currently are used to generate side-band frequencies from the "sync" output apertures are relatively inefficient and require relatively complex electronic apparatus. These techniques may require the use of sharp cut-off filters, local oscillators, balanced mixers, synthesis of sinusoidal waves from the square waves, multiple phase-locked loops, phase shift circuits, or the generation of signals with exactly four times the frequency of the carrier of modulator waves. These complexities can make the circuitry susceptible to faults and can either severely limit the frequency range of operation of the circuitry or require the inclusion of active tuning circuitry.